guzzlehttp/guzzle security advisory

CVE-2026-55767: Dot-only cookie domains match all hosts in guzzlehttp/guzzle

CVE-2026-55767 is a medium-severity session fixation vulnerability in the PHP package guzzlehttp/guzzle (CVSS 5.8), published 23 June 2026. It is fixed in 7.12.1; earlier versions are affected. It can be exploited without an account.

medium 5.8 published 2026-06-23 · updated 2026-08-05

Affected versions: < 7.12.1
Fixed in: 7.12.1
Newest release: 8.3.1
Severity: Medium · CVSS 5.8
Type: Session fixation
CWE-346, CWE-1286
Exploited in the wild: Not on the CISA KEV list
checked 2026-11-04

Is your guzzlehttp/guzzle version affected?

Run composer show guzzlehttp/guzzle in your project, then enter the version. We check it against this and every other known guzzlehttp/guzzle advisory.

Who can exploit CVE-2026-55767

Access: No account. Anyone who can send a request to an app that uses the affected code.

CVSS vector (scored by GitHub): CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:N

Your app is only exposed if it uses the affected part of guzzlehttp/guzzle. The advisory below says which part that is. If you are not sure, upgrade anyway: it costs less than finding out.

How to fix CVE-2026-55767 in guzzlehttp/guzzle

Find the installed version: composer show guzzlehttp/guzzle. If you never required it yourself, composer why guzzlehttp/guzzle shows which package pulls it in.
Update it: composer update guzzlehttp/guzzle --with-all-dependencies. You need 7.12.1 or newer.
If Composer will not go that high, your composer.json constraint is holding it back. Widen it (for example "guzzlehttp/guzzle": "^7.12.1" if you can take that version), or upgrade the package that requires it.
Run composer audit. It reads the same advisory data and should no longer list CVE-2026-55767.
Commit composer.lock and deploy it. On the server, run composer install --no-dev from the lock file, not composer update.

If you cannot upgrade yet

General hardening for session fixation issues while the upgrade waits. None of it replaces the fix.

Regenerate the session id at login and logout.
If your app does not use the affected feature, note that in your audit, but still plan the upgrade.

The CVE-2026-55767 advisory

Guzzle is an extensible PHP HTTP client. Prior to 7.12.1, CookieJar incorrectly accepts cookies with a dot-only Domain attribute and whitespace-padded variants. SetCookie::matchesDomain() removes leading dots from the cookie domain, normalizing dot-only values to the empty string; SetCookie::validate() only rejected a strictly empty domain, so these cookies could be stored and the empty normalized domain was treated as matching any request host. An attacker-controlled origin that an application requests with a shared cookie jar can therefore set a cookie that Guzzle later sends to unrelated hosts using the same jar. This may allow cookie injection or session fixation against downstream services, depending on how those services interpret the injected cookie. This vulnerability is fixed in 7.12.1.

Description from the CVE record, published by GitHub (the CVE Numbering Authority), via the NVD entry for CVE-2026-55767. Advisory title: “Dot-only cookie domains match all hosts”.

References

GitHub security advisory GHSA-cwxw-98qj-8qjx github.com
Fix commit 7f537cd github.com
Pull request #3653 github.com
Release 7.12.1 github.com
github.com github.com

Other guzzlehttp/guzzle advisories

high CVE-2026-69246: Guzzle: Noncanonical host can bypass host-based checks 2026-08-03
medium CVE-2026-69245: Guzzle: Noncanonical cookie domain keeps subdomain scope 2026-08-03
medium CVE-2026-67354: Guzzle: URI fragments disclosed in redirect Referer headers 2026-07-20
medium CVE-2026-67355: Guzzle: Host-only cookie scope is not preserved 2026-07-20
medium CVE-2026-67353: Guzzle: Unbounded response cookies risk denial of service 2026-07-20
medium CVE-2026-59883: Guzzle: Cookie Disclosure and Injection via IP-Address Domains 2026-07-20

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Where this data comes from

Affected and fixed versions: the Packagist security advisories API (GitHub Advisory Database and FriendsOfPHP), the same data composer audit uses. Synced 2026-11-03.
Description, score, weakness and references: NIST National Vulnerability Database (NVD), checked 2026-11-04. This product uses data from the NVD API but is not endorsed or certified by the NVD.
Exploited-in-the-wild status: the CISA Known Exploited Vulnerabilities catalog, checked 2026-11-04.
Compiled by Daniel Lasek at Laskyr from the sources above. The summary, “who can exploit it”, “how to fix it” and hardening sections are our plain-English reading of the record and are generated from it. The quoted description is the record's own text. Spot an error? Tell us.